Amalthea Mystery: How Did a Snowball Get So Close to Jupiter?
By Emily Lakdawalla
10 June 2005

The Jupiter system is a very orderly place. The giant planet has four large moons and a few smaller ones orbiting it in nearly circular paths. Just like the planets in the Solar System, Jupiter's moons decrease in density systematically as you go out. Io is close to Jupiter and made almost entirely of rock. As you travel outward past Europa, Ganymede, and finally Callisto, the moons have higher and higher proportions of water ice, so become less and less dense. It is so orderly that scientists have always assumed that a little object called Amalthea, which is Jupiter's innermost moon and the next largest after Europa, must be a rocky body, as dense as or denser than rocky Io. But they were wrong.

A team led by John Anderson of the Jet Propulsion laboratory has recently completed a two-year project to analyze Galileo data on Amalthea and made a surprising discovery. Not only is Amalthea not rocky, it is less dense even than water; according to the team's calculations, it is about 82% (plus or minus 9%) the density of the slightly dirty water ice typical of icy bodies in the Solar System. "We expected something perhaps asteroidal in density, or a solid rock object, which would have been much higher," Anderson says. "Amalthea just doesn't fit the pattern at all."

Why is the pattern so important? Because the orderly march of decreasing density out from Jupiter has been an important ingredient in models for how Jupiter formed. The outer moons of Jupiter (and also Saturn) have such low density. But these outer moons are all likely to have formed elsewhere in the Solar System, and been captured into their eccentric orbits around the giant planets. "You would think Amalthea was captured, because it doesn't fit the pattern with the other satellites," Anderson said. "But it has such a regular orbit. The orbit is almost circular and in the equatorial plane of Jupiter, so it looks like something that formed with Jupiter."

The measurement of Amalthea's density was derived from painstaking analysis of the radio signals sent by Galileo to the Earth as the spacecraft flew by Amalthea on November 5, 2002. The tiny gravity of Amalthea bent Galileo's course just slightly, changing its velocity by a mere few millimeters per second, and that change in velocity showed up as a minute Doppler shift in the radio signal that Galileo broadcast to Earth. Anderson and his team determined the mass of Amalthea from this Doppler shift in Galileo's signal, and found it to be unexpectedly small. Anderson and his team are the same people who discovered the anomalous acceleration of the Pioneer spacecraft through Doppler tracking.

If Amalthea is less dense than water, it pretty much has to have a composition of water ice with lots of open pores within it, like a snowball. And Amalthea's orbit is so close to Jupiter that the heat of the giant planet's formation would have vaporized any water so close to it, preventing it from condensing into a moon. That's why there's no water at Io, some at Europa, and more at Ganymede and Callisto -- because with increasing distance from Jupiter, the ambient temperature was lower when the system formed, so more water was available to condense at Callisto's orbit than Io's. There is no way an icy Amalthea could have formed where it is now.

"It has to be captured from somewhere," Anderson said. That means that it had to form outside the orbit of at least Io and probably farther out than that, and then somehow its orbit was perturbed to bring it in close to Jupiter, past the Galilean satellites in its way. The problem, Anderson says, is "there is no known mechanism for doing that. People haven’t been working that problem because they assumed it formed with Jupiter" as a rocky body.

Anderson's discovery means that scientists have to go back to the drawing board to figure out how the Jupiter system formed. Either Amalthea did not form at its current orbit, or it formed much later than all the other moons, after the system cooled; in either case, current theories for the formation of the Jupiter system don't stand up. It may well take another Jupiter mission to explain this mystery!

Mr. Christopher Go was invited on March 21, 2006 to be a member of the go_christopher.jpg (51417 bytes)Jupiter research team of the Hubble Space Telescope (HST). The tight schedule of activities of HST allows a limited amount of time for Jupiter. The Jupiter team is responsible for the activities for this limited period. Mr. Go was chosen for his achievements and expertise. He will not only provide continuous imaging from Cebu but will analyze results and direct research.

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Mr. Go belongs to a small group of very active amateur astronomers in the Philippines. You can check him out here:

Clouds on Jupiter have been caught swapping places, changing their shapes and colors as lower clouds move up and higher clouds sink.

Jupiter is wrapped in cloudy strips of yellows, browns and whites, created by winds at various latitudes blowing in different directions. Propelled by speedy winds, the clouds whip around the planet at hundreds of miles per hour.

High elevation clouds reside in "belt" regions of the Jovian atmosphere. Belt clouds are darker than those flying in the relatively low-elevation "zone" regions of the atmosphere. The winds in belts and zones flow in opposite directions.

Between March 25 and June 5, NASA's Hubble Space Telescope witnessed zones darkening into belts and belts lightening and transforming into zones. Clouds rapidly changed shapes and sizes.

NASA's Pluto-bound spacecraft, New Horizons, recently surfed a long tail of charged particles trailing behind Jupiter. Observations from that wild ride revealed enormous bobbing bubbles of charged particles, or "plasma," and showed that the structure of the planet's tadpole-shaped "magnetotail" is surprisingly varied.

The findings, detailed in two reports in the Oct. 9 issue of the journal Science, could help scientists understand the protective magnetic environment surrounding Earth and other planets.

Posted: Wed Dec 05, 2007 2:12 pm Post subject: The fine line between stability and instability -- when do g

University College London
5 December 2007

The fine line between stability and instability -- when do gas giants reach the point of no return?

Planetary scientists at UCL have identified the point at which a star causes the atmosphere of an orbiting gas giant to become critically unstable, as reported in this week’s Nature (December 6). Depending upon their proximity to a host star, giant Jupiter-like planets have atmospheres which are either stable and thin, or unstable and rapidly expanding. This new research enables us to work out whether planets in other systems are stable or unstable by using a three dimensional model to characterise their upper atmospheres.

Tommi Koskinen of UCL’s Physics & Astronomy Department is lead author of the paper and says: “We know that Jupiter has a thin, stable atmosphere and orbits the Sun at five Astronomical Units (AU) - or five times the distance between the Sun and the Earth. In contrast, we also know that closely orbiting exoplanets like HD209458b - which orbits about 100 times closer to its sun than Jupiter does - has a very expanded atmosphere which is boiling off into space. Our team wanted to find out at what point this change takes place, and how it happens.

“Our paper shows that if you brought Jupiter inside the Earth's orbit, to 0.16AU, it would remain Jupiter-like, with a stable atmosphere. But if you brought it just a little bit closer to the Sun, to 0.14AU, its atmosphere would suddenly start to expand, become unstable and escape. This dramatic change takes place because the cooling mechanism that we identified breaks down, leading to the atmosphere around the planet heating up uncontrollably.”

Professor Alan Aylward, co-author of the paper, explains some of the factors which the team incorporated in order to make the breakthrough: “For the first time we’ve used 3D-modelling to help us understand the whole heating process which takes place as you move a gas giant closer to its sun. The model incorporates the cooling effect of winds blowing around the planet - not just those blowing off the surface and escaping.

“Crucially, the model also makes proper allowances for the effects of H3+ in the atmosphere of a planet. This is an electrically-charged form of hydrogen which strongly radiates sunlight back into space and which is created in increasing quantities as you heat a planet by bringing it closer to its star.

“We found that 0.15AU is the significant point of no return. If you take a planet even slightly beyond this, molecular hydrogen becomes unstable and no more H3+ is produced. The self-regulating, ‘thermostatic’ effect then disintegrates and the atmosphere begins to heat up uncontrollably.”

Professor Steve Miller, the final contributing author to the paper, puts the discovery into context: “This gives us an insight to the evolution of giant planets, which typically form as an ice core out in the cold depths of space before migrating in towards their host star over a period of several million years. Now we know that at some point they all probably cross this point of no return and undergo a catastrophic breakdown.

“Just twelve years ago astronomers were searching for evidence of the first extrasolar planet. It’s amazing to think that since then we’ve not only found more than 250 of them, but we’re also in a much better position to understand

Slip-sliding away
By Susan Gaidos
May 23rd, 2008
The surface of one of Jupiter’s moons shifts position

Spin around quickly for a long period of time, and you’re likely to lose your balance and fall. Strangely, a similar thing can occur with orbiting bodies such as a planet. Spinning on its axis for millions of years, a planet’s surface features can shift position over time, upsetting its balance. If a major shift occurs, the planet might even tilt over.